Multiple glazing panel and manufacturing method therefor

ABSTRACT

A panel, and the manufacturing process therefor, having two sheets of glass or vitrocrystalline material joined in facing relation, that face of one sheet which constitutes an external face of the panel being chemically tempered to induce compressive stresses therein and being weakened to reduce its resistance to breakage due to flexures of the type which apply tensile forces thereacross.

United States Patent 191 Van Laethem et al.

[451 Apr. 2, 1974 1 MULTIPLE GLAZING PANEL AND MANUFACTURING METHOD THEREFOR [75] Inventors: Robert Van Laethem, Loverval; Pol

Baudin, Ransart, both of Belgium [73] Assignee: GlaverbelS.A.,

Watermael-Boitsfort, Belgium 22 Filed: June 1, 1971 21 Appl. No.: 148,334

[30] Foreign Application Priority Data May 21, 1971 Great Britain 16283/71 [52] US. Cl 161/117, 156/101,156/102, 156/106, 161/119, 161/164, 161/165,

[51] Int. Cl B32b 3/02, B32b 5/14, B32b 17/10 [58] Field of Search 161/1l6,119,120,121, 161/123, 164, 193, 199, 82,44, 45,183,

[56] 1 References Cited UNITED STATES PATENTS I 3,473,997 10/1969 Howitt 156/101 X 3,023,139 2/1962 Van Tetterode 65/31 3,616,839 11/1971 Burrin et a] 156/99 2,106,251 1/1938 Ludwig 161/45 3,522,143 7/1970v Motter 161/199 3,396,075 8/1968 Morris 156/102 3,282,772 ll/1966 Davis, Sr.... 156/102 1,748,080 2/1930 Reece 156/153 3,630,812 12/1971 Bruckner et al.. 161/125 3,317,297 5/1967 Ray 65/31 1,999,562 4/1935 Galey 65/65 R Primary Examiner--Daniel J Fritsch Attorney, Agent, or FirmSpencer & Kaye [57] ABSTRACT forces thereacross.

31 Claims, 15 Drawing Figures .Ilalll INVENTORS. Robert Van Loerhem Pol Bauclin ATTORNEYS Fig.7.

PATENTEU APR 2 I974 Pmmmm 21914 380L423 INVENTORS,

Robert Von Luethem Pol Buudin ATTORNEYJI PATENTEUAPR 21974 3,801,423

- sum 3 UF 4 I I l I I 5 I INVENTORS Robert Von Loerhem Pol Boudin Q BY E a MULTIPLE GLAZING PANEL AND MANUFACTURING METHOD THEREFOR BACKGROUND OF THE INVENTION This invention relates to processes of making a panel composed of a plurality of sheets in facing relationship, including a sheet of glass or vitrocrystalline, including vitroceramic, material providing an exterior face of such panel. The invention also relates to panels formed by such processes.

As will become clearer from the present disclosure, the sheets of such panels can be of any material, a substantial portion of which is constituted by a vitreous phase. The proportion of the material constituting such phase need only be large enough to enable modifications in the properties of the vitreous phase to significantly influence the properties of the sheet as a whole.

It is known to increase the tensile strength of glass by thermal or chemical tempering. The tempering treatment sets up compressive stresses in external layers of the glass. Such compressive stresses confer on the glass sheets mechanical properties which are advantageous for various purposes. In particular, the compressive surface stresses influence the tensile strength and the breakage characteristics of the glass sheets in ways which are of value for various practical uses of the sheets, tag. for their use as glazing material.

Unfortunately, while it is possible by a tempering treatment to give the glass sheets mechanical properties which are required for various purposes, all of the manifold effects of tempering are not always compatible with the particular product specifications to be complied with so that a compromise has to be made. For example, the tempering of a sheet of glass alters its breakage characteristics in the sense that the sheet breaks into smaller pieces than do untempered glass sheets, and at the same time the tempering increases the tensile strength of the sheet so that it is more resistant to breakage by flexing forces. The tempering treatment is therefore not wholly beneficial in cases where one but not another of these effects is required.

Theproblem arising from the interdependence of different properties is particularly marked in the case of panels incorporating a sheet of glass in facing relationship to another sheet of glass or other material because the mechanical properties of the panel are then determined by a larger number of parameters.

SUMMARY OF THE INVENTION It is a primary object of the present invention to overcome, or at least substantially reduce, these drawbacks and difficulties.

Another object of the invention is to provide a greater selection of the properties of such panels.

Still another object of the invention is to permit the properties of such panels to be made to conform more closely to that desired with regard to a wide variety of criteria.

A further object of the invention is to modify the effect of tempering on the sheets of such panels in a controlled manner.

The present invention provides a process of making a panel composed of a plurality of sheets in facing relationship, including a sheet of glass or vitrocrystalline material providing an exterior face of the panel, which process enables certain properties which have hitherto been regarded as interdependent to be independently varied so that a wider range of performance requirements can be satisfied.

According to the present invention, a process of making a panel composed of a plurality of sheet components in facing relationship, including a sheet of glass or vitrocrystalline material, is carried out by subjecting that sheet, hereafter referred to as the first sheet, to an ion diffusion treatment to induce compressive stresses in external layers of the sheet material, the sheet components of the panel are assembled so that one side of the first sheet provides an exterior panel face, and at a stage before, between or after the ion diffusion treatment and the assembly operation, the first sheet is subjected, at least locally, on at least the one side thereof, to a treatment, hereafter referred to as a weakening treatment, which reduces the resistance of the first sheet in at least one region thereof to breakage due to a flexure of the type which applies tensile forces to the external layers which are at the one side of the first sheet and which are, or become, compressively stressed due to the ion diffusion treatment.

The term vitrocrystalline material, where used in this specification, denotes a material produced from glass by a treatment which causes one or more crystalline phases to appear in the glass while leaving sufficient vitreous phase or phases at the surfaces of the material to permit compressive surface stresses to be induced therein by an ion diffusion treatment.

On its face, the treatment of a sheet of glass or vitrocrystalline material to cause any lowering of its tensile strength has no proper place in a process of fabrication in which the sheet is subjected to chemical tempering, this latter treatment being one which has always been regarded as a toughening treatment, i.e., precisely a treatment for increasing the tensile strength of the sheet. Nevertheless, it has been discovered, as part of the present invention, that a chemical tempering treatment and a treatment to reduce a tensile strength factor of the sheet can be performed on one and the same sheet with important advantageous consequences.

The process according to the invention enables a panel to be made which has an advantageous combination of properties. In particular, the yield strength, or tensile load level under which the first sheet will yield, by breaking, is less than the value necessary to overcome the compressive stresses which are due to the tempering treatment. At the same time, due to the production of such compressive stresses by chemical tempering, if the sheet does break, it divides into relatively small blunt pieces. Moreover, the yield strength of the first sheet can be controlled independently of the compressive stressing. Consequently, a wide range of product specifications can be achieved.

The extent to which the resistance of the first sheet to breakage under tensile loading is affected by the weakening treatment can be chosen in any given-process, taking into account the contrary effect or tendency of the selected chemical tempering treatment and the particular circumstances in which the panel is to be used.

The chemical tempering treatment can be of any known kind based on the diffusion of ions into glass or vitrocrystalline material from a contacting medium, preferably in molten state. Thus the tempering may be achieved by causing ions in exterior layers of the glass or vitrocrystalline material on at least one side of the first sheet to be replaced by larger ions deriving from a contacting medium, while the temperature of such layers is too low to permit complete stress relaxation in the sheet to occur during the time. The ion exchange may for instance involve the replacement of sodium ions in exterior layers of the glass by potassium ions deriving from the contacting medium. Alternatively, the tempering may be achieved by causing ions in exterior layers of the glass or vitrocrystalline material on at least one side of the sheet to be replaced by ions which confer on the exterior layers a lower coefficient of thermal expansion, such substitution occurring at a temperature sufficiently high to permit stress relaxation to occur. The exterior layers then become compressively stressed when the sheet cools. The ion exchange in this type of chemical tempering treatment may for example be an exchange of sodium ions in exterior layers of the glass for lithium ions deriving from the contacting medium.

The chemical tempering treatment may be one in which a diffusion of ions into exterior layers of the glass or vitrocrystalline material occurs only at one side of the first sheet. This will indirectly cause compressive stressing of the external layers at the other side of the sheet.

Needless to say, the glass or vitreous phase of the vitrocrystalline material can be of any composition capable of being chemically tempered. The particular ion diffusion employed can be of any type capable of chemically tempering, i.e. inducing compressive surface stresses in the particular material employed. The diffusion treatment can also be of the cementation type involving substantially a movement of ions in only one direction.

The weakening treatment is preferably performed substantially uniformly over at least one region of the first sheet. A predetermined breakage characteristic is thus achieved with respect to forces of similar magnitude acting anywhere in that region. Such region or regions subjected to such weakening treatment may be situated within a central and/or at a peripheral part of the sheet.

For certain purposes it is advantageous for the weakening treatment to be performed over substantially the whole or at least the one side of the first sheet. The weakening treatment can then be more easily performed as part of an industrial process, there being no need to take measures to confine the treatment to a predetermined region or regions of the sheet area.

The weakening treatment can in some cases advantageously be performed only on the one side of the first sheet. The treatment can then normally be more easily and rapidly effected.

However, such weakening treatment can be performed on both sides of the first sheet if desired. If such weakening treatment is performed on both sides of the first sheet before the panel components are assembled, the treated sheet can be located in the panel with either side thereof exposed.

If diffusion of ions into the first sheet occurs only at one side thereof, it is preferable for that side to be employed as an exterior panel face and for the weakening treatment to be performed at least on that side of the sheet. If this condition is observed, a required predetermined resistance to breakage due to flexure in one direction can be more easily achieved.

One preferred type of weakening treatment is carried out by surface abrasion of the first sheet. This type of treatment is inexpensive and simple to perform. When performing a chemical tempering treatment on successive sheets, the yield strength thereby achieved shows a certain variation from one sheet to another. The performance of a weakening treatment by abrasion offers the important advantage that the resistance to breakage can be brought to a more uniform value from one sheet to another.

Abrasion can advantageously be effected using a particulate material having an average grain size of the order of 10 microns. With such abrasive substance. the weakening treatment can be performed easily without causing undesirable defects in the sheet, such as scratches visible to the naked eye.

Advantageously, the weakening treatment involves an abrasion of the first sheet with a powder composed substantially of iron oxide, cerium oxide, or alumina, or of a mixture of two or more of such substances.

According to another type of weakening treatment, the weakening is effected by surface scratching the first sheet. Preferably the scratches are made entirely or predominantly in one direction. The weakening effect is then greatest with respect to flexure forces extending along planes substantially normal to the direction of the scratches.

Before the weakening treatment, the first sheet, if it is a glass sheet, can have at least its one side subjected, at least locally, to a treatment, hereafter called a supplementary strengthening treatment, whereby the resistance of the glass to breakage by flexure is at least temporarily increased in at least one region of the sheet, such treatment being performed after chemical tempering of the sheet but before the weakening treatment thereof. The supplementary strengthening treatment has the effect of facilitating the attainment of a predetermined degree of weakening by the subsequent weakening treatment.

Advantageously, the supplementary strengthening treatment involves a chemical dissolution of a surface layer of the first sheet in at least one region thereof. This type of supplementary strengthening treatment permits the required strengthening effect to be obtained by a relatively brief contact of at least a given region or regions of the sheet with a dissolving medium which may, e.g., be in liquid or gaseous form.

The supplementary strengthening treatment can advantageously be performed by contacting the first sheet in the region or regions to be treated, e.g., by spraying, with an acid medium containing fluorine ions, for example an acid medium essentially composed of a compound such as ammonium fluoride or an aqueous solution containing hydrofluoric acid.

It has been found that the use of an acid medium containing fluorine ions not only gives better results in regard to mechanical properties as before referred to, but also increases the resistance of the glass to iridescence. This advantageous result is particularly marked when use is made of an aqueous solution containing hydrofluoric acid and sulphuric acid, e.g., solution containing 6 percent by volume of hydrofluoric acid and 6 percent by volume of sulphuric acid. With such an acid medium, satisfactory results can be obtained very quickly, e.g., by a treatment lasting only a few minutes.

Preferably the supplementary strengthening treatment by means of an acid medium is performed at a temperature of between 0 and 80C. In this temperature range the speed of dissolution of the sheet material at a surface thereof can be accurately controlled.

In the case of a glazing panel, e.g., a vehicle windshield, it is desirable to ensure that the resistance of the first sheet to breakage due to flexure forces tending to stretch, or tension, the one side thereof is such that, in the completed panel, the first sheet will break under the impact of a person against the opposite side of the panel at impact forces below the level at which serious injury, e.g. to the skull, becomes probable. Such level will normally not be higher than that which corresponds to a tensile loading of 50 kg/mm at the one side of the first sheet as measured on a disc 11 cm in diameter. That maximum value may apply over only a given region or regions of the sheet, but it preferably applies over the whole area of the sheet. The safeguard against serious bodily injury is then realized irrespective of the position at which impact against the panel may occur.

In the fabrication of a panel according to the invention, the first sheet can be secured in spaced relation to a second main sheet component, i.e. a second strength member in sheet form, the first and second sheets being directly connected only at marginal portions thereof, and the space between the central parts of the sheets being empty or containing a gaseous medium or a filling material. The properties of the panel are then influenced by such inter-sheet space or filling. Moreover, in the event of flexure of the second sheet so that its inner face becomes convexly curved or more convexly curved than originally, the first sheet is not influenced by that flexure until it reaches a certain value which depends on the spacing between the sheets.

However, according to preferred embodiments of the invention, the sheet components of the panel are secured together over their entire area to form a laminate. Such embodiments give a product in which'the behavior of the panel under flexing forces is always influenced by the properties of the component sheets acting in combination.

Particular importance is attached to embodiments of the invention in which the first sheet is assembled in facing relationship to a second main sheet component i.e., a second strength member in sheet form, which second sheet can, in the completed panel, be flexed by an amount to impose flexing forces on the first sheet sufficient to break the latter sheet. The panel then has a very advantageous combination of properties. If the panel is held at its margins and becomes subjected to flexing forces imposed against the second sheet, the first sheet will break at a given load while the said second sheet is still capable of further elastic flexure. This particular property of the panel is of importance for avoiding concussive injury to a person who may make impact with the panel, e.g. in the case that it is used as a vehicle windshield. Moreover, when the first sheet breaks it divides into rather small and blunt fragments,

which are not a serious accident hazard, e.g. to other road users in the case that the panel is used in a road vehicle.

Another very important consequence of the weakening treatment of the first sheet used in the above embodiment, or other embodiments, is that the maximum loading forces, as aforesaid, which the sheet can support without breaking, is not dependent on the magnitude of the compressive stresses induced therein by the chemical tempering treatment. Moreover the first sheet can be of a thickness such that it has a high resistance to breakage due to impact by small hard objects such as stones, without losing the advantage that the sheet will yield to loading forces of relatively small magnitude acting against the other side of the panel.v

In one form of panel according to the invention, said first sheet is assembled in facing relationship to a second main sheet component made of a plastic material. A plastic sheet can have a very high flexibility, which is an advantage for shock absorption. For forming a laminate, such sheet of plastic can be directly bonded to the first sheet of glass or vitrocrystalline material without an intervening bonding layer.

In preferred embodiments of the invention, the first sheet is assembled in facing relationship to only one other strength member in sheet form and that second sheet is also a sheet of glass or vitrocrystalline material. The panel is then of simple construction, and makes good utilization of the advantageous properties of glass or vitrocrystalline material. In such embodiments the sheets are preferably secured together to form a laminate.

In optimum-forms of such embodiments, the first sheet of glass or vitrocrystalline material is secured in facing relationship to a second sheet of glass or vitrocrystalline material to form a laminated panel and the relative strengths of the first and second glass or vitrocrystalline sheets, considered independently of each other, are such that, notwithstanding the weakening of the first sheet by a weakening treatment, the resistance of that first sheet to breakage due to a flexure which subjects its one side to tension forces is higher than the resistance to breakage of the second sheet due to a flexure which subjects to tensioning forces the side of the second sheet which in the completed panel faces the first sheet. This condition is not inconsistent with the condition that, in the completed panel, the first sheet should break under flexing forces, imposed against said second sheet, of a magnitude insufficient to break said second sheet, because in the completed panel the component sheets behave as a monolithic structure up to the moment of breakage of the first sheet, and under any given flexing force the one side of the first sheet is subjected to higher tensile loading than the side of said second sheet facing the first sheet. The advantage of giving the first sheet the said higher breakage resistance is that when the panel is used as a glazing panel, e.g., as a windshield or in a door, the first sheet is capable of absorbing a higher proportion of impact energies before it breaks.

The relative strengths of the first and second sheets can be attained by giving the first sheet an appropriately greater thickness than the second sheet. Alternatively, if in order to keep the weight of the panel below a certain value, or for other reasons, the thickness of the first sheet must be less than that which would be necessary, apart from other factors, for attaining the higher strength, such strength can be attained by the chemical tempering treatment or by this treatment and appropriate selection of the thickness of the sheet.

Advantageously, the first sheet is secured to a second sheet, which is of glass, and prior to the sheet assembly operation the second sheet is subjected to a treatment, hereafter called a rectifying treatment, which reduces the incidence on at least one portion of its inner face, i.e. the face which is to be directed toward the first sheet, of flaws liable to cause stress concentrations when the inner side of such sheet is subjected to tensioning forces.

Such rectifying treatment gives the second sheet greater resistance to breakage due to flexure in the direction which places the inner face under tension. While it is very advantageous to subject the whole of the inner face of said second sheet to a rectifying treatment, an improvement in the resistance of the sheet to breakage by flexure can often be achieved by subjecting a particular zone or zones of such sheet to such a rectifying treatment, and notably by subjecting at least one marginal zone of the sheet face to such rectifying treatment.

Such a rectifying treatment can advantageously be performd by heating at least one zone of the sheet so as to cause a tire polishing thereof. Such a treatment can be performed by pre-heating the sheet to a temperature of between 400 and 450C and then exposing the zone or zones to be rectified for a relatively brief period, for example from a few seconds to a few minutes, to a higher temperature, e.g., a temperature substantially in the range of 600 to 850C. It has been found that this type of rectifying treatment can be performed with little or no tendency for the optical properties of the sheet to be impaired, which is a very important consideration when the sheet is to be used in a panel which is to form a windshield.

Another very satisfactory type of rectifying treatment involves chemical dissolution of a surface portion of the sheet. This type of treatment is easy to perform. For example, the treatment can be carried out by a simple contact of the sheet with a solvent medium by sprinkling, spraying or immersion, there being no need to use a large or expensive installation. Moreover the degree of rectification can be easily controlled by varying the concentration of the solvent medium.

Such a dissolution treatment can advantageously be performed by contacting the surface to be treated with an acid medium containing fluorine ions. It has been found that acid media containing fluorine ions act quite rapidly to give very satisfactory results and surfaces treated by such media show an improved mechanical strength and an improved resistance to iridescence.

When performing rectification by chemical dissolution, it is preferable to dissolve a surface layer, in the treated zone or zones, having a thickness at least equal to the deepest surface flaw present in such zone or zones and liable to cause a stress concentration. In that manner, all flaws in the treated zone or zones become eliminated and the mechanical strength of the sheet material is made substantially uniform over the treated zone or zones.

Preferably when performing a rectification treatment by chemical dissolution, at least one portion of the sheet being treated is substantially shielded from the solvent medium by a protective layer. Such protective layer may, e.g., be a layer of paraffin wax, a peelable varnish or a thin fluid film, for instance a film of water which is caused to flow along the surface portion or portions to be protected during the rectification treatment. By taking such a protective measure, the rectification treatment can be confined to a given zone or zones, thus reducing the amount of solvent used.

Moreover, it has been found that certain optical flaws in a glass sheet may be rendered more visible if a surface of the sheet is subjected to acid treatment, particularly to treatment with an acid medium containing fluorine ions, unless that surface is subsequently contacted by a layer of organic material. There is therefore sometimes an advantage, particularly when making windshields, for the outwardly facing side of the second sheet to not be subjected to a rectifying treatment.

The resistance of the second sheet of glass or vitrocrystalline material to breakage due to flexures in a direction towards the first sheet can also be improved by compressively stressing the external layers of glass or vitrocrystalline material of that sheet at least at the inner face thereof.

The compressive stressing of such second sheet can be achieved by thermal tempering. Preferably, however, such compressive stressing of the second sheet is achieved by chemically tempering the second sheet, using any of the chemical tempering processes hereinbefore referred to as suitable for application to the first sheet. The use of an ion diffusion treatment for compressively stressing the exterior layers of the second sheet achieves the advantage that in the event such second sheet breaks, it will break into small blunt pieces which do not involve a high risk or personal injury, or damage to the remainder of the panel. Moreover, when an ion diffusion treatment is used for compressively stressing exterior layers of both of the first and second sheets, the manufacturing plant can be laid out and operated more conveniently.

Another manner of increasing the resistance of the second sheet to breakage due to flexure in a direction towards the first sheet is to incorporate the second sheet into the panel while it is in a state of elastic flexure in one or more planes and in a direction such that the second sheet is being compressively stressed at its inner side. This method is extremely simple. The second sheet can be held in an elastically flexed condition by a frame, or by the other component sheet or sheets of the panel or by the combined action of such other sheet or sheets and a frame.

By way of example, the second sheet of the panel may be constituted by a naturally curved glass or vitrocrystalline sheet which is secured at its convex side to a first sheet which is naturally flat or which has a natural curvature less pronounced than the second sheet, so that in the completed panel the second sheet is held, against the elastic recovery'forces therein, by the first sheet in a flat condition or at a curvature less than its natural curvature.

The first sheet may itself be held, by a frame component and/or by the second sheet, in a state of elastic flexure such that its outer face is in a state of tension or reduced compression. Such flexure has the effect of lowering the flexing load level at which the first sheet will break.

Any two or all three of the above-described methods of increasing the resistance of the second sheet to breakage by flexure which subjects its inner face to tensile stress, viz: a rectifying treatment, a chemical tempering treatment, and elastic flexure, can be applied to one and the same second sheet.

In certain embodiments of the invention, the first sheet is secured in facing relationship to the second sheet of glass or vitrocrystalline material which has itself been chemically tempered and is itself subjected, at least locally, on the side thereof which in the completed panel faces away from the first sheet, to a weakening treatment which reduces the resistance of the second sheet in at least one region thereof to breakage due to a flexure in the direction which imposes tensile forces in the external layers which are at that side of the second sheet. Such weakening treatment may be performed before or after the panel sheets are assembled.

By performing a weakening treatment on the outwardly 1 any case preferable for the weakening treatment of the second sheet to be performed substantially uniformly over at least one region thereof, disposed in a central and/or peripheral part of such sheet.

Advantageously, the first and second chemically tempered sheets are subjected to weakening treatments at portions of the faces which are directed away from each other in the completed panel and which are in registry with one another. The resistance to breakage of one or more portions of the panel under loading forces acting against either side of the panel can thus be readily and accurately determined without the necessity for subjecting the whole of the outwardly facing sides to weakening treatments. For instance, in a door glazing panel produced according to the invention, those portions of the outer faces of the door where strong accidental impact is most likely to occur may be given a lower mechanical strength. As another example, in the case of a panel constituting a vehicle windshield, the outwardly facing sides of the first and second sheets can be weakened in a region or regions where the head of the driver or of a front seat passenger is likely to strike the screen in the event of hard braking or headon collision.

The particulars hereinbefore given relating to the manner in which the weakening treatment can be performed on the first sheet are also relevant for performing a weakening treatment on the second sheet.

Advantageously, in the case that the second sheet is of glass, this sheet is at least locally subjected, before a weakening treatment thereof, to a supplementary strengthening treatment, at least at the side thereof which is to face away from the first sheet. Such supplementary strengthening treatment may be performed in any of the ways hereinbefore described in relation to the supplementary strengthening of the first sheet.

In a process in which the first and the second sheets are subjected to a weakening treatment, it is often advantageous for the weakening treatment of both sheets to be performed before the panel sheets are assembled. This procedure is particularly advantageous when identical weakening treatments have to be performed on such sheets because they can be treated successively in a continuous processing line.

In other embodiments of a process according to the invention in which both the first and second sheets are subjected to a weakening treatment, the weakening treatment of the two sheets is performed after the panel sheets have been assembled. This procedure is more particularly advantageous when the panel is not flat, for example when the panel has a relatively small radius of curvature, and when aligned portions of the two sheets in the panel have to be weakened. In such cases the required weakening of the sheets can be effected simply by passing the panel once through a machine or installation which treats the opposed outer panel faces simultaneously.

In the production of a laminate by a process according to the invention, the first sheet can be secured to a second strength member in sheet form, by means of an intervening organic sheet or sheets, e.g., a sheet of organic material. By means of such an intervening sheet or sheets, first and second sheets of glass or vitrocrystalline material can be secured together to form a transparent or translucent laminate. Moreover, the intervening sheet or sheets can serve to retain individual pieces of the first or second sheet in place in the event of breakage.

Such an intervening sheet can, e.g., be a sheet of thermoplastic material. Particularly satisfactory interlayer materials are polyvinyl butyral, e.g. in the form of a sheet 0.76 mm in thickness, and certain polycarbonates of bis-phenols which may be used together with,

for example, polyacrylate adhesives.

The invention also includes a panel composed of a plurality of sheet components in facing relationship, including a sheet of glass or vitrocrystalline material in external layers of which compressive surface stresses have been produced by a chemical tempering treatment, one side of such sheet providing an exterior face of the panel, according to the invention, in at least one region of the glass or vitrocrystalline sheet, which corresponds to the first sheet referred to herein, the tensile strength of such sheet, expressed in terms of the tensile force, imposed by flexing the sheet, which can be supported without breakage by the compressively stressed external layers at the one side of the sheet, is made less than the tensile force necessary for reducing such compressive stresses to zero.

Such a panel possesses an important combination of properties. In particular, the tensile strength of the first sheet, expressed as above referred to, is not only determined by the magnitude of the compressive stresses produced by tempering, and at the same time the existence of the stressesdue to chemical tempering causes the sheet, if itdoes break, to divide into relatively small blunt, i.e. non-cutting, pieces. In the case of a glazing panel, the tensile strength of such first sheet can, for example, be low enough to ensure that if a person should collide against the other side of the panel, the sheet will yield before the impact force reaches a value at which serious internal personal injury is probable.

Advantageously, the tensile strength of the first sheet in at least one region thereof has a uniform lower value than the tensile force necessary for reducing the compressive stresses therein to zero. Such uniformity is considered to exist when the tensile strength of various samples of the sheet within that region, tested in the herein concerning the advantages afforded by the corresponding process features.

Advantageously, the one side of the first sheet bears surface abrasions or scratches in at least one region of the sheet. The presence of scratches is of particular value if they are entirely or primarily in one direction. In the case of a vehicle windshield, it is preferable for the scratches to be wholly or predominantly in a direction normal to the longitudinal axis of the windshield.

Advantageously, at least one region of the one side of the first sheet has grooves or scratches having a depth of less than microns. Such scratches or grooves are consistent with good transparency or other optical properties of such sheet.

It is very advantageous, in the case that the first sheet is of glass, for at least one region of the first sheet to contain fluorine ions in external layers of the one side thereof.

For glazing panels, e.g. Windshields, which may be subjected to impact by a person, it is advantageous for the tensile strength of at least one region of the first sheet, expressed in the terms hereinbefore specified, to correspond to a value of less than 50 kg/mm as measured on a disc ll cm in diameter.

The first sheet may be secured in facing spaced relationship to a second main sheet component, the first and second sheets being directly connected only at the margins of the panel. The space between the central portions of the sheets can be empty or may contain a fluid medium, e.g., dry air or other gaseous medium, or a filling material. The panel may, e.g., incorporate a filling material having good heat insulating properties. According to a particular embodiment, the sheets are assembled by means of a metal ribbon or ribbons located between the sheet margins.

In panels of the most preferred type according to the invention, the sheet components of the panel are secured together over their entire area and form a laminate.

According to particularly important embodiments of panels according to the invention, the first sheet is in facing relationship to a second main sheet component of the panel, which second sheet can be flexed to impose flexing forces on the first sheet sufficient to break that sheet. Such a panel has a combination of properties which are of notable value as hereinbefore described in relation to processes according to the invention, particularly in the case of a glazing panel, e.g., a vehicle window or windshield.

In a very simple form of panel according to the invention, the first sheet of glass or vitrocrystalline material is in facing relationship to a second sheet component, such second sheet being a sheet of plastic material. The plastic sheet can be bonded to the first sheet with or without an intervening bonding layer to form a laminate, or the plastic sheet and the first sheet may be in spaced relation.

Preferably, the panel consists of the first sheet of glass or vitrocrystalline material and only one other strength member in sheet form, that second sheet being also a sheet of glass or vitrocrystalline material. Particular preference is given to such panels in the case where such sheets are secured together to form a laminate. Such embodiments are particularly important, for reasons hereinbefore stated, in the case where the second sheet has been chemically tempered and is more elastically flexible than the first sheet.

In optimum embodiments of the invention, the first sheet of glass or vitrocrystalline material is secured in facing relationship to a second sheet of glass or vitrocrystalline material and the sheets constitute plies of a laminate. The relative strengths of the first and second sheets are such that the second sheet can be flexed to impose flexing forces on the first sheet sufficient to break the latter sheet, but they are such that the first and second sheets were tested independently of each other, the resistance of the first sheet to breakage due to a flexure subjecting its one side to tensioning forces would be higher than the resistance to breakage of the second sheet due to a flexure subjecting to tensioning forces the side which faces the first sheet.

The relative strengths of the first and second sheets can be determined by individually peripherally supporting a plurality of identical sheets in turn and dropping a rounded object weighing 10 kg onto each sheet frofn a height which is progressively increased from one test to the next and noting in the case of each sheet the dropping height at which the sheet breaks. This test is very suitable for panels to be used as vehicle windshields.

According to a preferred feature, the panel includes, as the second sheet, a sheet of glass, the inner face of which is, over at least one region of the sheet, substantially free from flaws liable to cause stress concentrations when tensioning forces are applied to the inner face of such sheet. Advantageously, the external layers of the second sheet at the inner side thereof contain fluorine ions in at least one region of such sheet.

In panels with optimum properties, the second sheet of glass or vitrocrystalline material has compressively stressed external layers at least at its inner side. Such compressive stressing is preferably due at least in part to chemical tempering of such second sheet.

Compressive stressing of the external layers of glass or vitrocrystalline material at the inner side of the second sheet may also be achieved by sheet flexure as hereinbefore described and in certain panels according to the invention the first sheet is in facing relation to a second sheet of glass or vitrocrystalline material which is held in a state of elastic flexure in a direction such that the external layers at the inner side of the second sheet are compressively stressed. Preferably, the second sheet is held elastically flexed by the first sheet. However, the second sheet can be held flexed by means of a frame, or by means of the first sheet and a frame.

Advantageously, the first sheet is held by a frame and/or by a second sheet, in a state of flexure such that the one side of the first sheet is in a state of tension or reduced compression.

The first sheet of glass or vitrocrystalline material may be in facing relationship to a second sheet of glass or vitrocrystalline material which has itself been chemically tempered, the tensile strength of at least one region of the second sheet, expressed in terms of the tensile force, imposable by flexing the sheet, which can be sustained by the compressively stressed external layers, at the side of the second sheet facing away from the first sheet, being less than the tensile force necessary for reducing such compressive stresses to zero. Advantageously therefore, the second sheet bears surface abrasion or scratches in at least one region at the side of such sheet which faces away from the first sheet.

This type of panel is particularly advantageous when there is a risk of each side of the panel being on some occasion subjected to an impact force under circumstances such that a very high resistance to such force would have undesirable effects. A glazing panel of a glass door is a case in point.

Advantageously, in an embodiment in which the first and second sheets have been subjected to a weakening treatment, this treatment serves to weaken the sheets in zones which are opposite each other, i.e. in registry, in the panel. A glazed door or a windshield can for example have its first and second sheets weakened in at least one region of the panel where impact by a person, such as might cause serious personal injury, is liable to occur. Such weakened region may have a mechanical strength which is accurately predetermined. I A panel according to the invention can advantageously be composed of the first sheet of glass or vitrocrystalline material secured to a second strength member in sheet form by means of an intervening organic sheet or sheets, e.g., a sheet of organic polymeric material such as a sheet of polyvinyl butyral or a high molecular weight polycarbonate of a bisphenol which may be cemented by means of a polyvinyl acrylate adhesive to the sheets between which it is sandwiched. Such an intervening sheet can serve to'hold individual pieces of the first and second sheets in the event of breakage, and a stable assembly of two sheets, for example of glass, can be readily produced. Also,,the panel can, if required, have a very good transparency and other advantageous optical properties, such as a lowoptical distortion.

Preferably, the first and second sheets are secured together by means of an intervening sheet of organic material which resists penetration or tearing if one or each of the first and second sheets is broken by the impact of a rounded object weighing kg and dropped from a height of about 620 cm. If a panel satisfying that test is used as vehicle windshield, there is but small risk of the head of a driver or passenger penetrating the windshield upon being thrown thereagainst.

In a panel according to the invention wherein the first sheet of glass or vitrocrystalline material is in facing relationship to a second main sheet component which is also of glass or vitrocrystalline material which has also been chemically tempered, it is advantageous for the first sheet to be thicker than the second sheet. The benefits of such a panel when used as a glazing panel, for example a windshield, are best realized when the panel is installed with the thicker sheet facing toward the outside, assuming that forces tending to flex the panel are most likely to act against the inside of the panel, whereas the outside of the panel is most likely to be struck by small hard objects causing indentations. Preferably, the first sheet has a thickness in the range of 1.5 to 4 mm and the second sheet has a thickness in the range of l to 2.5 mm.

A selection of properties appropriate to the locations of the different sheets can often be realized if the compressive stressing of the thicker sheet due to chemical tempering is substantially equal to or less than the compressive stressing of the thinner sheet due to chemical tempering.

In any embodiment of a process or panel according to the invention, the first sheet may be naturally flat or naturally curved in one or more planes.

It is to be understood that the sheet of glass or vitrocrystalline material which constitutes the first sheet may be coated before or after the assembly operation to produce the panel. For example, after the assembly operation, the face of the first sheet which constitutes an external panel face may be coated with a coating layer, e.g. an anti-reflection layer.

in the embodiments covered in the foregoing description, one'side of the first sheet of glass or vitrocrystalline material which is subjected to a weakening treatment constitutes an exterior panel face. By way of modification, the panel may incorporate a sheet which covers the one face, provided that the covering sheet has negligible resistance to flexure, i.e., provided it is not a strength member. For example such a covering sheet may be a thin plastic foil which is used for protective or coloring purposes.

Accordingto another aspect of the present invention, there is provided a panel composed of a first sheet of glass secured by means of an intervening organic sheet to a second sheet of glass, each of the first and second sheets having been chemically tempered. According to novel features of the invention, the first sheet is thicker than the second sheet, and the inherent resistance of thefirst sheet to breakage by flexure in a direction such that the flexure imposes tensioning forces on its side which is remote from the second sheet is higher than the inherent resistance of the second sheet to breakage by flexure which imposes tensioning forces on the side of that sheet which faces the first sheet, but the second sheet of the assembly can be flexed to impose flexing forces on the first sheet sufficient to break the first sheet. The inherent resistance of the first or second sheet to breakage is the resistance of that sheet to breakage when tested in isolation from the other sheet.

Such a panel has a very important combination of properties which make it very suitable for use as a glazing panel in a building or vehicle, and particularly as a vehicle windshield. When used as a windshield with the thicker glass sheet facing the outside of the vehicle, the panel affords a high degree of safety to vehicle occupants in the event of impact of an occupant against the inside of the windshield and in the event of the windshield being struck from the outside by small hard objects, e.g. pieces of gravel, or by larger objects, e.g. large stones which may fall against the windshield while being loaded into a truck.

The panel may be flat or may be curved in one or more planes.

According to preferred embodiments of such panel, the first sheet of glass has a thickness at least 1.25 times that of the second sheet of glass.

Preferably, the thicker glass sheet presents an external face of the panel, but it may be covered by a sheet which is not a strength member.

Preferably, the thinner glass sheet also presents an external face of the panel.

The inherent strengths of the sheets of glass of different thicknesses may be tested, for example by individually peripherally supporting identical sheets and dropping a rounded object weighing 10 kg onto each sheet from a height which is progressively increased from one test to the next until the sheet breaks.

In a panel according to the invention, composed of sheets of glass of unequal thickness and different inherent resistances to breakage as above referred to, any of the features hereinbefore referred to in connection with the invention in its first aspect can be present, for example: an all-over or localized weakening of the first sheet or of the first and second sheets; a second sheet with a rectified inner face or an inner face which is compressively stressed in part by reason of such sheet being held in the panel in a state of elastic flexure; the presence of fluorine ions in the exposed face of the first sheet and/or in the inner face of the second sheet; and an intervening sheet of thermoplastic material resisting penetration by an impacting rounded object as hereinbefore referred to in the event that the second sheet breaks under the impact.

In a panel according to the second aspect of the invention as above referred to, the thicker glass sheet preferably has a thickness in the range of 1.5 to 4 mm and the thickness of the thinner glass sheet is preferably in the range of l to 2.5 mm. When observing such thickness ranges, it is easy to produce a panel which is both strong and flexible enough to avoid excessive impact shock to a person thrown head first against the side of the panel at which the thinner sheet is exposed. Moreover, such a panel is of fairly low weight, which is a feature of importance in the case of panels to be used as Windshields in competition cars.

The invention also includes processes of making a panel according to the invention in its second aspect. Such process may have any of the process features hereinbefore referred to.

Considering now the mechanical strength tests permitting to measure the rupture stress to flexure and being carried out on discs having a diameter of 11 cm as described in the present specification, it is useful to describe briefly the way to perform the said tests:

a. the said discs are disposed freely on a ring-shaped holder;

b. the loading of the said discs is carried out with the help of a measuring-machine Instron (Instron is a trade mark name); the surface of applying the said loading is circular, central in relation to the surface of the disc BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of one form of panel according to the invention.

FIG. 2 isa cross-sectional view taken along the line 2-2 of FIG. 1. a

FIG. 3 is a cross-sectional view of another embodiment of a panel according to the invention.

FIG. 4 is a cross-sectional view of a further embodiment of a panel according to the invention.

FIG. 5 is a perspective view of one embodiment of a windshield according to the invention.

FIG. 6 is a cross-sectional view taken along the line 6-6 of FIG. 5.

FIG. 7 is an elevational view of another embodiment of a windshield according to the invention.

FIG. 8 is a cross-sectional view taken along the line 88 of FIG. 7.

FIG. 9 is a cross-sectional view of the components of one embodiment of a windshield according to the invention prior to assembly.

FIG. 10 is a cross-sectional view of the embodiment of FIG. 9, after assembly.

FIG. 11 is an elevational view of one embodiment of a window pane] according to the invention.

FIG. 12 is a cross-sectional view taken along the line 12-12 of FIG. 11.

FIG. 13 is a cross-sectional view of another embodiment of a window panel according to the invention.

FIG. 14 is a cross-sectional view of the components of another window panel according to the invention, prior to assembly.

FIG. 15 is a cross sectional view of a window pane] formed of the components illustrated in FIG. 14 and after assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the detailed description of the figures to be presented below, the reference numerals, l, 2, 3 and 4 are utilized to designate respective faces of the components of the various embodiments of the invention. The illustrated embodiments will be described with reference to a series of specific examples. All of the embodiments of the invention are multilayer articles having a flat or curved configuration and generally constitute door or window panels, or vehicle Windshields. Therefore, reference throughout the following description to panels or Windshields is to be construed as a reference to such multilayer articles.

Example I The article illustrated in FIGS. 1 and 2 is a translucent panel intended to be utilized as a closure panel, such as a door for a building, having a high degree of fire resistance. For safety reasons, such a panel should satisfy numerous requirements, and in particular the following:

a. the panel should have a high degree of fire resistance so as to provide a maximum impediment to the propagation of fire from one side of the panel to the other;

b. the mechanical strength of the panel should be such that, if a person becomes imprisoned within the area closed off by the panel, the panel can be broken to free such person by striking the panel, or otherwise subjecting it to shocks, either from within or without the enclosure;

0. the mechanical strength of the panel should be such that, in case a person should accidentally run into it, he will not be in danger of suffering serious head injuries (cranial trauma).

To construct a panel satisfying these requirements, a group of six panels were each formed from two identi-' cal sheets 5 and 7, each measuring 2 m X 2.5 m X 0.003 m and formed from a vitroceramic material having the following composition, by weight:

slo, 65.5% A p, 26.0% up 4.0% m0 4.5%

Sheets 5 and 7 were joined together by means of a sheet 6 of polyvinyl butyral. In the finished panel, the faces 1 and 2 constitute surfaces of the sheet 5 intended to be disposed toward the interior of the enclosure when the panel according to the invention is utilized as a closing panel for such enclosure, while the faces 3 and 4 constitute surfaces of the sheet 7 intended to be directed toward the exterior of such enclosure. It should be noted, however, that in this specific case, the panel is composed of two sheets having identical characteristics, so that the side of the enclosure toward which each sheet is directed is of little importance.

Before assembling the sheets to form the panels, six sheets and six sheets 7, as well as 12 discs having a diameter of 11 cm and of the same material and same thickness as the sheets,5 and 7, were treated by immersion for 24 hours in a bath of potassium nitrate maintained at a temperature of 450C. After washing and drying these sheets and discs, the surface rupture stress due to flexure of six of the treated discs was measured and it was noted that the highest rupture stress measured was 91 kg/mm and the lowest measured rupture stress was 74 kg/mm Then, one face of each of the six remaining treated discs and one part of the faces 1 and 4 of the sheets 5 and 7, respectively, intended to constitute the exterior faces of the panels, were subjected to an abrasion treatment such that the value of the rupture stress due to flexure of those faces was made no greater than 50 kglmm The abrasion treatment was performed by using sand whose grains had an average diameter of p, the sand being applied to the entirety of one face of each of the six discs and to a rectangular part 8 and 9 of each of the facesl and 4 of sheets 5 and-7, respectively. The rectangular part of each face 1 and 4 had an area of 3.6m and the rectangular portions 8 and 9 were disposed in registry with one another in such a manner that the lateral sides of each of the rectangular parts 8 and 9 were 0.1m from 2.5m long sides of the respective sheets 5 and 7, while thelower edge of each rectangular part was spaced 0.4m from the lower edge of each respective sheet. The rupture stress due to flexure of the six discs which had oneface submitted to the abrasion was measured and it was noted that the lowest strength value measured was 4 l kg/mm while the highest value measured was 50 kg/mm Then, one sheet 5 which has been abraded as described above was assembled with one sheet 7 which had been abraded in the same manner, and the sheets were joined together by means of a high impact polyvinyl butyral sheet having a thickness substantially equal to 0.76mm to form a panel having the structure illustrated in FIGS. 1 and 2. It should be noted that, in order to facilitate an understanding of the invention, the effect of the abrasion treatment has been greatly exaggerated in FIG. 2.

The mechanical strength tests mentioned above revealed that the resulting panels, after installation, perfectly satisfied the requirements enumerated earlier.

Similar panels were also constructed from sheets 5 and 7 of the same vitroceramic material, and having the same dimensions, as those described above, but which differed from the above-described panels by the fact that only one face of one of the panels, i.e., either the face 1 or the face 4, intended to form one of the exterior faces of the panels, was chemically tempered by being maintained in contact for 24 hours with potassium nitrate maintained at 450C and the abovedescribed abrasion treatment of faces l and 4 was performed before the chemical tempering.

sio, Na,0 13% CaO 10% A1,,o 2% MgO 3% the remainder of the composition being constituted by minor amounts of compounds such as K 0, Fe 0 Each of these panels also included a sheet 7 identical in composition with the sheet 7 described above. Both of the sheets 5 and 7 had dimensions identical with those of the sheets described above.

Before joining the sheets 5 and 7 together by means of a high impact polyvinyl butyral intervening sheet having a thickness of 0.76 mm, the faces 1 and 4 of sheets 5 and 7 were subjected to a strengthening treatment by being maintained for 24 hours in contact with molten potassium nitrate at a temperature of 450C, and to an abrasion treatment identical with that disclosed above and utilizing sand whose grains have an average diameter of 1011.. After the treatment, it was noted that the last sheet 5 was no more flexible than the vitroceramic sheet 7.

After fabrication, such panels were installed as enclosure panels arranged so that the glass sheet 5 was directed toward the interior of the enclosure with which they were associated. These panels also satisfied the requirements enumerated earlier.

Example 2 A lot of twelve sheets of soda-lime glass measuring 1.5m X 2 m X 3 mm, and having the following composition, by weight:

sio Al,0, 2% Na,0 l2% CaO 12% MgO 2% the remainder being constituted by impurities, was subjected to an ion exchange treatment. Sodium ions initially present in the glass were replaced by lithium ions in a bath containing 2% LiNO and 98% NaCl and at a temperature of 580C. The treatment lasted 20 minutes. The sheets were then withdrawn from the bath, cooled and cleaned.

.MXQQQ discs having a d saste t 1.1. 9 arid of the same thickness and composition as the abovedescribed 12 sheets were subjected to the same ion diffusion treatment. The rupture stress due to flexure applied symmetrically with respect to the center of the discs was measured for six of the 12 discs and different values were obtained for each of the discs. The lowest measured value was 14 kg/mm and the highest measured value was 19 kg/mm.

Then, the six remaining discs simultaneously had one face of each subjected to an abrasion treatment utilizing alumina and the abrasion treatment was halted when the rupture stress due to flexure of one of the discs became equal to kg/mm The rupture stresses due to flexure of the other discs was then measured and it was noted that the lowest measured value was 13 kg/mm while the highest measured value was 16 kg/mm. One face of each of the sheets was abraded by a treatment identical to that carried out on the discs.

The sheets were then joined together, two by two, by means of a sheet of high molecular weight polyvinyl chloride and utilizing a low molecular weight polyvinyl chloride as an element aiding the adherence between the high molecular weight polyvinyl chloride and the glass. This resulted in panels of the type illustrated in FIG. 3, wherein the glass sheets 10 and 11 are joined together by means of a polyvinyl chloride sheet 12, with the abraded faces 1 and 4 of the sheets constituting the outer surfaces of the panel. This panel is intended to be utilized as a glass door and it has been noted that in case the door suffers a shock due to being run into by a person, the glass sheets do not cause any head injuries because they break before the shock impact reaches a dangerous level.

According to one modification of the abovedescribed procedure for forming the panel illustrated in FIG. 3, instead of the entirety of the faces 1 and 4 of the glass sheets being subjected to an abrasion treatment, only one part of these faces was subjected to such a treatment, either before or after the diffusion treatment and assembly of the panel. According to this modification, the abraded portions of faces 1 and 4 are in registry, i.e. they are coextensive with one another, and are disposed at the location having the greatest probability of being struck by a person moving against the panel. This arrangement is particularly advantageous when the panels are utilized as glass doors, in which case there is a greater probability that the central region of the surface of the faces 1 and 4 will be struck by a person moving toward the door.

Example 3 FIG. 4 illustrates a panel according to the invention, one component of which is constituted by a rectangular glass sheet 15. This sheet measures 1 m X 0.5 m X 0.006 m and has the following composition, by weight:

SiO, 73%

Na,0 13% CaO 9% o, 3% MgO 1% the remainder of the sheet being constituted by impurities. To give this sheet its desired mechanical properties, it was subjected to the following treatment.

Several of these sheets were immersed, together with 24 control discs having a diameter of 1 1 cm and of the same thickness and composition as the sheets, for 24 hours in a bath of potassium nitrate maintained at 460C and containing 0.2 percent, by weight, potassium carbonate. Then, after having been washed and cooled, the sheets and discs were immersed for 3 minutes in an aqueous bath containing 7 percent, by volume, hydrofluoric acid and 7 percent, by volume, sulfuric acid at a temperature of C. As a result of this treatment fluorine ions have been introduced into the 6 external layers of the glass articles. After having then been washed in distilled water and dried in an isopropryl alcohol vapor, the surface rupture strength with respect to flexure of a group of six treated discs was measured in the manner indicated above and it was noted that the lowest measured value was 1 l0 kg/mm while the highest measured value was 130 kglmm Then, all of the sheets and the remaining discs has one face of each simultaneouslysubjected to an abrasion by means of a cerium oxide powder and the abrasion operation was halted when the rupture stress due to flexure of one of the discs reached a value of 49 kg/mm Then, the value of the rupture stress due to flexure for the remaining 17 discs was measured and it was noted that the lowest rupture stress value was 47 kg/mm, while the highest rupture stress value was 50 kg/mm An improved resistance to iridescence was also noted.

Each of the sheets thus treated was joined, by means of a polyvinyl butyral sheet 14, as illustrated in FIG. 4, to a sheet 13 having a thickness of 1.2 mm in such a manner that the abraded surface of sheet 15 constituted the outer face 4 of the resulting panel. The sheet 13 is more flexible than the 6 mm thick sheet 15 and is composed of a soda-lime glass which was treated by immersion for 24 hours in a potassium nitrate bath maintained at 460C and containing 0.2 percent, by weight, potassium carbonate. The resulting panels were intendedfor use as closures for the control cabins of cranes, the 6 mm thick sheet 13 being disposed toward the exterior.

According to one modification of the procedure which has just been described, panels comparable to those described were formed of a glass sheet 15 having a thickness of 6 mm and joined, by means of a polyvinyl butyral sheet 14, to a glass sheet 13 having a thickness of 1.2 mm. Before joining, the sheets 15 and 13 were chemically tempered by immersion for 24 hours in a bath of potassium nitrate maintained at 460C and containing 0.2 percent potassium carbonate. Besides the fact that the face 4 of sheet 15 underwent the abrasion treatment described above in the present example in such a manner as to cause the rupture stress of that face due to flexure to be less than 50 kg/mm the face 2 of glass sheet 13 was subjected to a treatment serving to at least partially remove surface flaws, which would be sources of stress concentrations if forces were applied to the sheet.

The surface flaw removal treatment was carried out on six sheets 13 before assembly of the panel illustrated in FIG. 4. The treatment was carried out before the chemical tempering with potassium nitrate, described above, and was performed on the face 2 of the six sheets. The flaw removal treatment was carried out at a temperature of 20C and involved the application to the face 2 of the sheetsof an aqueous solution having the following composition:

8.8 liters of water 0.6 liter of H 80,

0.6 liter of HF.

The solution was maintained in contact with the sheets for about 60 minutes and served to remove a 60p. thick layer of each face 2, which corresponded substantially to 1.5 times the depth of the most serious flaws noted on the face before treatment. The other face 1 of sheets 13 had been preliminarily coated with paraffin to prevent substantially any contact with the solution. After cleaning of the sheets 13, they were joined, as described above, to corresponding sheets 15 and it was noted that the resulting panels presented an improved resistance to penetration due to a shock applied against face 1.

For another group of six sheets 13 intended to be joined to six sheets 15, a different flaw removal treatment was carried out on the face 2 of each sheet 13 prior to the above-described chemical tempering treatment. This flaw removal treatment was of the type commonly known as fire polishing. For this purpose, the sheets 13 were preheated to 450C. Then, they were placed above a flame formed by passing propane and compressed air through a porous refractory piece. The sheets were maintained, with their faces 2 directed toward the flame, for 45 seconds and the temperature at the faces 2 reached a value of 660C. The sheets were then cooled progressively in a furnace initially maintained at 450C. Then, the resulting sheets 13 were joined to respective ones of sheets prepared and treated in the manner described above to form the panels illustrated in FIG. 4. It was also noted that these panels presented an improved resistance to penetration due to a shock against face 1.

Example 4 Several laminated glazings having the form illustrated in FIGS. 5 and 6 were each formed from a sheet 16 and a sheet 18 of curved, chemically tempered glass, the sheet 16 having a thickness of 1.2 mm and the sheet 18 having a thickness of 3.2 mm. The sheets were constituted by soda-lime glass having an ordinary composi tion. The glazings were obtained by bonding the sheets 16 and 18 to respective opposite faces of a sheet 17 of high impact polyvinyl butyral having a thickness of 0.76 mm. The glazings were intended to be utilized as Windshields mounted in such a manner that the 3.2 mm thick sheet 18 would be disposed toward the exterior of the vehicle, while the sheet 16, which is more flexible than the sheet 18, is disposed toward the interior of the vehicle.

Before assembly of the glazings, the sheets 16 having a thickness of 1.2 mm and the sheets 18 having a thickness of 3.2 mm, aswell as two groups of 12 control discs having a diameter of 11 cm, one group of discs having a thickness of 1.2 mm and the other group of discs having a thickness of 3.2 mm, and all of the discs being of the same composition as the sheets 16 and 18, were subjected to an ion diffusion treatment by immersion for 24 hours in a potassium nitrate bath maintained at a temperature of 450C. After this potassium nitrate treatment, the rupture stress due to flexure was measured for six discs of each group and it was noted that the lowest value measured was 66 kg/mm while the highest value measured was 108 kg/mm. Then, all of the sheets having thicknesses of 1.2 mm and 3.2 mm, as well as the remaining discs, simultaneously had one face of each subjected to an abrasion by means of a powder composed of cerium oxide and alumina. After this abrasion, the rupture stress due to flexure was measured for the 12 discs, six of each thickness, and it was noted that the lowest measured value was 45 kg/mm while the highest measured value was 49 kg/mm In addition, a group of sheets 16 and a group of sheets 18 were subjected, after the abrasion treatment and before assembly, to a test of their mechanical strength with respect to flexure to verify if the strength of sheets 18 was greater than that of sheets 16. These tests established, as expected, that the sheets 18 had a greater mechanical strength than the sheets 16. Then, each sheet 16 was joined to a sheet 18 to form a panel,

the sheets being arranged so that the abraded faces 1 and 4 of the sheets constituted the outer surfaces of the resulting windshield. It was determined that in case the windshield suffered a shock due to the impact of a human head, or an object simulating a human head, against face 1 of the windshield, the exterior sheet 18 broke within satisfactory limits, from a biomechanical point of view, i.e., the rupture stress of face 4 due to flexure did not exceed 50 kg/mm Example 5 A series of sandwich-type Windshields were each fabricated of two sheets of chemically tempered glass having respective thickness of 1.4 mm and 2 mm and formed from a glass having substantially the following composition, by weight: 2

SiO, 72%

Na,0 10% CaO 14% A120, 9%

the remainder being constituted by minor amounts of compounds such as MgO, Fe,O These Windshields were fabricated by cementing or otherwise bonding the two sheets to opposed faces of a sheet of high impact polyvinyl butyral having a thickness of 0.76 mm.

These Windshields are intended to be mounted in automobiles in such a manner that the 2 mm thick sheet, represented by the sheet 18 of FIG. 6, is disposed toward the exterior of the vehicle, while the 1.4 mm thick sheet, constituted by the sheet 16 of FIG. 6, is disposed toward the interior. Before assembly, the series of 1.4 mm thick sheets, the series of 2 mm thick sheets, and two groups of control discs 11 cm in diameter, one group of discs having a thickness of 1.4 mm and the other group of discs having a thickness of 2 mm, both groups of discs being of the same compositions as the sheets, were subjected to an ion diffusion treatment by immersion for 24 hours in a bath of potassium nitrate maintained at a temperature of 450C. After this treatment, the rupture stress due to flexure was measured for 12 discs, six discs of each thickness, and it was determined that the highest measured value was kg/mm and the lowest measured value was 56 kg/mm Then, one face of each of the 2 mm thick sheets, the face constituting the face 4 of the arrangement illustrated in FIG. 6, as well as one face of the six treated control discs having a thickness of 2 mm, were subjected to an abrasion utilizing alumina powder. After this abrasion, the rupture stress due to flexure was measured for the six abraded control discs, and it was observed that the lowest value measured was 39 kg/mm and the highest value measured was 47 kg/mm It was also observed, by comparative tests, that the flexure strength of the chemically tempered 2 mm thick sheets, corresponding to the sheet 18 in the arrangement of FIG. 6, was after the abrasion treatment still greater than the strength of the chemically tempered 1.4 mm thick sheets, corresponding to the sheet 16 of FIG. 6.

Windshields having the form illustrated in FIG. 6

were then formed from one each of the 1.4 mm thick shields were subjected to mechanical strength tests and it was observed that they satisfied the biomechanical requirements for the safety of persons within the automobile, i.e., in every case the tensile rupture stress of the face 4 was no greater than 50 kg/mm According to a modification of the above procedure, Windshields were produced in the manner described above in the present example, with the exception that the abrasion treatment was performed after joining together the sheets 16 and 18 and it was noted that the resulting Windshields also satisfied the biomechanical requirements for the safety of the occupants of the automobile.

Example 6 FIGS. 7 and 8 illustrate another embodiment of an automobile windshield according to the invention composed essentially of two sheets and 22, each formed from a soda-lime glass having the following composition, by weight:

s10, 71% Na,0 12% CaO 14% A1,o, 2%

the remainder being constituted by minor amounts of other compounds.

The sheet 22 has a thickness of 1.5 mm and is. intended to be directed toward the interior of the automobile, while the sheet 20 has a thickness of 3.2 mm and is intended to be directed toward the exterior of the automobile. Before being joined together to form the windshield, these sheets were treated in the following manner:

Firstly, the sheets corresponding to sheet 20 of FIGS. 7 and 8 were cut by tracing around the periphery of the sheet, 1 cm from the edge and only on the face 4, a groove 19 having a depth of 0.03 mm. This constituted a weakening treatment. After this weakening treatment, the sheets 20 as well as the sheets 22 which did not undergo a weakening treatment, were subjected to a strengthening treatment by immersion for 24 hours in a bath of potassium nitrate maintained at a temperature of 450C. After washing and drying, the sheets 20 and 22 were joined together by means of a high impact polyvinyl butyral sheet 21 having a thickness of 0.76

The resulting Windshields were subjected to mechanical stresses and it was observed that this type of windshields satisfied the biomechanical requirements for the safety of persons within the automobile, i.e., in all cases the tensile rupture stress due to flexure of the face 4 was no greater than 50 kg/mm.

Further, another group of sheets 22 and a group of sheets 20, these sheets having been treated in the manner described above, i.e., the sheets 20 having undergone a chemical tempering with potassium nitrate maintained at 450C and the sheets 20 having, before the chemical tempering, been provided with grooves 19, were subjected to tests of their mechanical strength with regard to flexure and it was noted that the sheets 22 were more flexible than the sheets 20 and that the sheets 20, after having been weakened by the provision of grooves and strengthened by chemical tempering,

had a mechanical strength with regard to flexure which was still greater than the corresponding mechanical strength of the sheets 22. Example 6a A batch of windscreens illustrated by FIGS. 9 and 10 was made. The windscreen components, immediately prior to the assembly step, were substantially the same as the components of the windscreens made as described in Example 6, immediately prior to the assembly of those components, with the exception however that the sheet 22 of each windscreen had a natural curvature somewhat more pronounced than the natural curvature of the sheet 20 of the screen. The components of the windscreen shown in FIGS. 9 and 10 and the groove or scratch in the thicker glass sheet thereof bear the same reference numerals as the corresponding parts in FIGS. 7 and 8. For assembling the sheets to form a windscreen shown in FIG. 10, the sheet 22 had to be elastically flexed so that in the panel the inner side of the sheet 22 was compressively stressed due to the flexure. The laminated sheets could be held flat as shown in FIG. 10 by means of a frame (not shown) or the sheets could remain at a curvature intermediate the natural curvatures of the sheets 20, 22, each sheet being sustained at that curvature by the elastic recovery forces stored in the other sheet. In either case, due to the fact that the inner side of sheet 22 was compressively stressed due to flexure, the sheet 22 was able to withstand higher impact forces acting against face 1 of the panel so that there is less risk of the windscreen being penetrated by a body in the event of it making high energy impact against such face. The windscreens fully met the required biomechanical specifications for avoiding serious risks of injury to a driver or passenger. The tensile strength of face 4 of the windscreens was at most 50 kg/mm". In order to prevent the groove 19 in face 4 from causing optical faults, e.g., due to accumulation of dust in such groove, a sheet 24 of polymethylmethacrylate 0.5 mm in thickness and having an optical quality at least as good as that of the glass sheet 20 was glued to that sheet.

Example 7 A series of panels having the form illustrated in FIGS. 11 and 12 were formed by cementing a stretched acrylic plastic sheet 26 having a thickness of 12.7 mm to a sheet of glass 25 having a thickness of 2.2 mm. The sheet 25 was made of a glass having the following composition, by weight:

SiO, 72% N320 14% C210 9% A1,o 3% M30 1% the remainder being impurities.

Before being joined to the sheet 27, the glass sheet 25 was strengthened by immersion for 24 hours in a potassium nitrate bath maintained at 460C. To this end, a group of 12 sheets 25, as well as 12 samples in the form of discs having a diameter of 11 cm, and having the same thickness and composition as the sheets 25, were immersed for 24 hours in a bath of potassium nitrate at 460C.

After this treatment, the mechanical strength relative to flexure was measured for a group of six discs and it was observed that the lowest measured value was kg/mm while the highest measured value was kg/mm Then, six of the sheets 25 and six of the discs, which had been chemically tempered, each had one face subjected to an abrasion by means of a powder composed of alumina and cerium oxide and the abrasion treatment was halted when the mechanical strength with regard to flexure of one of the discs reached a value of 48 kg/mm Then, the five remaining discs which had been abraded were subjected to mechanical strength tests with respect to flexure and it was observed that the lowest rupture stress measured was 40 kg/mm and the highest value measured was of the order of 49 kg/mm*.

For a group of three sheets 25, the abrasion took place before assembly, while for a group of three other sheets 25, the abrasion took place after assembly. No noticeabledifference was observed in the six resulting assemblies formed from these two groups of sheets 25. In addition, for the assemblies having the form illustrated in FIGS. 11 and 12, it was observed that, in case of shock applied against face 1 of the assembly, the sheet 26 was more flexible than the sheet 25 and that the biomechanical safety conditions were satisfied if the face 4 of sheet 25 had been abraded in the manner described above, because the face 4 of sheet 25 then had a rupture stress with respect to tension of no greater than 50 kg/mm According to one modification of the procedure just described, it was also noted that it was possible to obtain assemblies of the type illustrated in FIGS. 11 and 12 which were satisfactory from the point of view of biomechanical requirements if grooves having a depth of the order of 0.05 mm were cut in the face 4 of sheet 25.

Example 8 I FIG. 13 illustrates another panel formed according to the invention, which could be utilized as a fire resistant panel. These panels are constituted by a sheet 28 of vitroceramic material measuring 2.1 m X 1 m X 0.006 m and formed from a vitro-ceramic material having the following composition, by weight:

SiO, 74.0% A |,0, 16.2% up 5.8% no, 4.0%

and a glass sheet 27 measuring 2.1 m X l m X 0.002 m and formed from a window glass of ordinary composition.

The sheet 28 is intended to be disposed'toward the exterior when the panel is utilized as a closure panel of a dwelling, a door of an enclosure or an elevator, or a window of an enclosure. The panel is assembled in the following manner. Around the periphery of the face 2 of sheet 27 and the face 3 of sheet 28 thin copper films 29 and 30, respectively, are disposed. The sheets 27 and 28 are then soldered together by means of a tin solder layer 31 which becomes bonded to the copper films 29 and 30. For purposes of illustration, the thicknesses of films 29 and 30 and of solder layer 31 have been greatly exaggerated. Air trapped within the space 32 between sheets 27 and 28 had been thoroughly dried before the space became completely sealed by the solder layer 31. According to one modification, similar panels were produced but, to improve the insulation and acoustic property of the panels, the space 32 was filled with glass wool.

Before assembly, the two types of sheets, i.e., the vitroceramic sheets 28 and the glass sheets 27, were subjected to the same chemical tempering treatment performed by immersion of the sheets for 24 hours in a bath of potassium nitrate maintained at a temperature of 460C. After this chemical tempering treatment and a washing and drying, the sheets were assembled as described above to form the panel illustrated in FIG. 13.

Subsequently, the face 4 of vitroceramic sheet 28 was weakened by cutting therein a groove 33 having a depth of the order of 0.05 mm and extending around the periphery of the face at a distance of 1 cm from the edge. With the face 4 weakened in this manner, it was noted that the panels presented numerous advantages, the principal ones of which are:

a. while retaining a good resistance to fire, the panels could be easily broken from the interior, which permitted easy evacuation of persons trapped in the enclosure closed by the panel;

b. while retaining a good mechanical resistance to flexure in case of a shock against sheet 27, which is much more flexible than the sheet 28, the sheet 28 absorbs a part of the energy received by the sheet 27 when the latter comes in contact with sheet 28, and sheet 28 breaks when subjected to a rupture stress inferior to the value at which head injuries could occur, if the shock were due to a person striking the panel, it being recalled that this critical value is of the order of kg/mm Example 9 FIG. 15 illustrates another type of hollow panel formed according to the invention and composed essentially of two sheets 34 and 35, both made of glass having the following composition, by weight:

SiO 71.0%

Na,0 16.0% C210 1 1.0% MgO 0.6% [0,0, 1.4%

The two sheets 34 and 35 had respective thicknesses of 1.5 mm and 3 mm and were joined together by means of a frame 36.

Two different types of panels comparable to that illustrated in FIG. 15 were constructed. To produce the first type of panel, the following procedure was employed. Firstly, the sheets 34 and 35 were initially given respectively different curvatures, as shown in FIG. 14, where the sheet 34 is flat and the sheet 35 is curved. The sheet 35 is intended to be directed toward the exterior of a building or room when the resulting panel is utilized as a closing panel and its curvature is such that its face 4 will be placed in a state of compression when the sheet 35 is forced into a flat configuration by the holding action of frame 36.

Before assembly, the sheets 34 and 35 were subjected to a chemical tempering treatment by immersing them for 24 hours in a potassium nitrate bath maintained at a temperature of 450C. Twelve control discs, six of each thickness, each having a diameter of 11 cm and formed from the same glass as the sheets 34 and 35 were chemically tempered together with those sheets. Subsequently, each sheet 35 was treated by being brought into contact for minutes with an aqueous solution maintained at a temperature of 20C and having the following composition:

8.8 liters of water 0.6 liter of H 80,

0.6 liter of HF It was observed that this treatment which introduced fluorine ions into the external layers of the glass sheets not only increased the mechanical strength of the sheets relative to flexure, but also the sheets had a better resistance to iridescence. Before assembly, the entirety of the face 4 of each sheet 35 and one face of each of the control discs having the same thickness were subjected to an abrasion treatment by means of an alumina powder whose grains had an average diameter of 15p. This abrasion treatment was halted when the rupture stress due to traction of one of the discs was measured to be 49 kglmm It was then noted, by mechanical strength tests, that for a group of six sheets 35 treated as described above, the value of the rupture stress of the sheets with respect to traction was between 45 kg/mm and 50 kg/mm As regards sheets 34, the face 2 of each of these sheets was subjected to a rectifying treatment such as a treatment utilizing hydrofluoric acid or the fire polishing treatment described above in Example 3. in both cases, the strength of face 2 was improved and each sheet 34 withstood a shock produced by a steel ball weighing 227 grams and falling from a height of 2.9 m onto face 1.

Then, each treated sheet 34 was assembled with a treated sheet 35 by means of a frame 36 in such a manner that the sheet 35 was caused to assume a planar configuration and it was noted that the resulting panels possessed numerous advantages, the principal ones of which are:

a. the face 4 presented a good resistance to indentation and to iridescence;

b. in case of a shock caused by a person striking face 1 of the panel, the sheet 34, being more flexible than the sheet 35, came to bear against the sheet 35 and the latter broke when the tensile stresses induced in the face 4 by sheet 34 reached a value of 50 kg/mm, so that it was possible to prevent serious head injuries.

According to a modification of the above procedure, the second type of panel was formed from a sheet 34 which was chemically tempered and slightly curved before assembly in such a manner that, during assembly, the sheet 34 was forced to assume a planar configuration so that compressive stresses were induced in its face 2'. This permitted the mechanical strength of the sheet due to flexure to be increased. According to the conditions imposed by the user, one can combine this treatment for increasing the mechanical resistance of the sheets 34 to flexure with a fire polishing treatment of the face 2, or with a treatment by an aqueous solution of hydrofluoric acid, or simply subject the face 2 to one of these treatments.

Example 10 Windscreens as shown in FIGS. 5 and 6 were made, each comprising two sheets 16, 18 of soda-lime glass having the following composition by weight:

SiO, 72.2% Na,0 16.4% CaO 9.4% MgO 0.6% Fe,0, 0.2% 0, 0.5% Na,so, 0.7

secured. together by an interposed sheet 17 of high impact polyvinylbutyral, 0.76 mm in thickness. The sheets 16, 18 were 1.2 mm and 2.0 mm in thickness respectively.

Before assembly, the glass sheets were separately subjected to an ion diffusion treatment by immersion in a bath of potassium nitrate at a temperature of 450C. After such ion diffusion treatment, measurements were made of the resistance of samples of the sheets 16, 18 to breakage by flexure placing their convex faces in tension. In other words each sample was flexed to determine the maximum tensile loading which could be imposed on face 2 or 4, as the case may be, by flexing the sample, before breakage of the sample occured. It was found that sheet 18 could withstand a higher tensile loading on its convex side than the sheet 16.

A batch of the windscreens was subjected to different mechanical strength and biomechanical tests as hereafter specified.

In one test each of 10 of the windscreens was subjected to impact by a body weighing 10 kg of rounded shape and having substantially the volume of a human head by dropping the body onto face I of the windscreen from a height of 620 cm while the screen was peripherally supported. In each case the sheet 16 shattered into small blunt fragments at the zone of impact, but in no case did the body penetrate or even rupture the polyvinylbutyral intersheet 17.

In another test, each of another ten of the windscreens was subjected to a similar impact by a rounded body weighing 10 kg dropped from a height of 620 cm, the body being covered with two chamois leathers to simulate human skin, and the intensity of the impact force sustained by the body and the gravity of the cuts in the leather skins were measured. This involves measuring the so-called combined index, defined as the sum of:

i. one hundredth of the weighted safety index defined by General Motors Corporation, viz:

f a dt where a deceleration expressed in multiples of g acceleration due to weight;

ii. laceration index having a value from 0 to 10 depending on the gravity of the cuts; 0 being the value assigned in the absence of cuts and 10 the value assigned to fatal cuts.

Thus the combined index is: f (a dt/ l 00) laceration index. For meeting the safety specifications in view, the value of the combined index should be less than 20.0. In the case of the batch of windscreens made according to the present example and subjected to the said test, the value of the combined index was in no case higher than 7.0.

A further batch of the windscreens made according to this Example were also subjected to the impact of pieces of hard gravel projected against face 4 of the screen at different speeds. It was found that in 90 percent of the tests in which pieces of the gravel were projected against the screens at km per hour the windscreens were apparently unaffected or at most a dent from 50 to I00 microns in depth appeared in face 4. In no case was there any cracks radiating from the indentation so that windscreens affected in that way during use would not have to be replaced.

A further batch of the windscreens made according to the present Example were subjected to flexure by the application of an increasing pressure against face 1 while the screen was peripherally supported. In each case, notwithstanding the higher inherent tensile strength of sheet 18 as shown by the tests on individual sheets 16, 18 as hereinbefore referred to, the sheet 18 broke when a certain flexing load was reached but at sio, 76%

Na,0 12% Ca() 10% Ago, 2%

and a sheet 17 of high impact polyvinylbutyral 0.76 mm in thickness. The glass sheets 16, 18 were 1.5 mm and 3 mm in thickness respectively. The sheets 16, 18 were subjected to identical ion diffusion treatment by immersion for 24 hours in a potassium nitrate bath kept a temperature of 450C and each sheet 16 was then secured to one of the sheets 18 by means of a sheet 17 of polyvinylbutyral.

The resulting windscreens were subjected to various mechanical strength and biomechanical tests and it was found that they satisfactorily met the required specifications for the safety of a vehicle driver or passenger. For instance, impact strength tests were carried out on several of the windscreens by allowing a rounded body weighing 10 kg and having substantially the volume of a human head to fall onto face I of the windscreen from a height of 620 cm. In each case the face 1 shattered into very small blunt fragments at the zone of impact but in none of the cases was the intersheet of polyvinylbutyral penetrated or even torn. In similar tests, the intensity of the shock received by the mass representing the human head was measured and it was found that with such impacts the weighted safety index defined by General Motors Corporation and corresponding to the formula:

1 (weighted safety index) :I a dt. where: a deceleration expressed in multiples of g acceleration due to weight, never reached the critical value of 1,000. This value of the weighted safety index is that at which head injuries become highly dangerous if not fatal.

Moreover face 4 of each of several of the windscreens was subjected to the impact of pieces of hard gravel projected against the screen at different speeds. In 90 percent of the tests in which the pieces of gravel were projected against the screens at 1 kg per hour, the windscreens were apparently unaffected or at most a dent from 50 to 100 microns in depth was formed in face 4. In no case were there any cracks radiating from the indentation so that windscreens affected in that way during use would not have to be replaced.

In addition, tests were made on six chemically tempered sheets 16 and six chemically tempered sheets 18, as used in the windscreens, to determine their respective resistances to breakage by flexure in a direction placing their convex faces (faces 2 to 4) in tension. It was found that the resistance of the sheet 18 was higher than the resistance of sheet 16. In other words, face 4 was able to withstand a higher tensile loading than face 2. The relative strengths of the said sheets were however such that when one of the completed windscreens was flexed in a direction such as to place faces 2 and 4 in tension by exerting increasing flexing pressure against the central zone of sheet 16 while the windscreen was peripherally supported, sheet 18 broke when a certain flexing load was reached while sheet 16 remained capable of further flexure.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. In a panel comprising a plurality of sheet components in facing relationship, at least one of said sheet components being a first main sheet of an at least substantially vitreous material which is chemically tempered to have compressive surface stresses in external layers at at least one face of said first sheet, which face constitutes an exterior face of said panel, the improvement wherein said first sheet is weakened at its said one face, whereby the tensile strength in at least one region of said first sheet, expressed in terms of the tensile force, imposable by flexing said first sheet, which can be sustained by the compressively stressed external layers at said one face of said first sheet is less than the tensile force necessary for reducing such compressive stresses to zero.

2. An arrangement as defined in claim 1 wherein the tensile strength of said first sheet in at least one region thereof is substantially uniformly lower than the tensile force necessary for reducing said compressive stresses therein to zero.

3. An arrangement as defined in claim 1 wherein the said tensile strength of said first sheet is substantially lower over the whole area thereof than the tensile force necessary for reducing said compressive stresses therein to zero.

4. An arrangement as defined in claim 1 wherein said one face of said first sheet bears surface abrasions in at least one region of said first sheet.

5. An arrangement as defined in claim 1 wherein said one face of said first sheet bears surface scratches in at least one region of said first sheet.

6. An arrangement as defined in claim 5 wherein said one face of said first sheet is provided in at least one region thereof with surface grooves having a depth of less than 5 microns.

7. An arrangement as defined in claim 1 wherein said first sheet is of glass and external layers of said first sheet at said one face thereof contain fluorine ions.

8. An arrangement as defined in claim 1 wherein said tensile strength of at least one region of said first sheet is less than 50 kg/mm as measured on a disc of the material of said first sheet 1 1 cm in diameter.

9. An arrangement as defined in claim 1 wherein there is a second main sheet constituting another one of said components, said first sheet is in facing relationship to said second sheet, and said first and second sheets are directly connected only at the margins of said panel.

10. An arrangement as defined in claim 1 wherein said sheet components of said panel are secured together over their entire surface area.

11. An arrangement as defined in claim 1 wherein there is a second main sheet constituting another one of said components, said first sheet is in facing relationship to said second sheet, and said second sheet is arranged to be flexed toward said first sheet, by a flexing force acting against the face of said second sheet which is directed away from said first sheet, to cause said second sheet to impose flexing forces on said first sheet 

2. An arrangement as defined in claim 1 wherein the tensile strength of said first sheet in at least one region thereof is substantially uniformly lower than the tensile force necessary for reducing said compressive stresses therein to zero.
 3. An arrangement as defined in claim 1 wherein the said tensile strength of said first sheet is substantially lower over the whole area thereof than the tensile force necessary for reducing said compressive stresses therein to zero.
 4. An arrangement as defined in claim 1 wherein said one face of said first sheet bears surface abrasions in at least one region of said first sheet.
 5. An arrangement as defined in claim 1 wherein said one face of said first sheet bears surface scratches in at least one region of said first sheet.
 6. An arrangement as defined in claim 5 wherein said one face of said first sheet is provided in at least one region thereof with surface grooves having a depth of less than 5 microns.
 7. An arrangement as defined in claim 1 wherein said first sheet is of glass and external layers of said first sheet at said one face thereof contain fluorine ions.
 8. An arrangement as defined in claim 1 wherein said tensile strength of at least one region of said first sheet is less than 50 kg/mm2 as measured on a disc of the material of said first sheet 11 cm in diameter.
 9. An arrangement as defined in claim 1 wherein there is a second main sheet constituting another one of said components, said first sheet is in facing relationship to said second sheet, and said first and second sheets are directly connected only at the margins of said panel.
 10. An arrangement as defined in claim 1 wherein said sheet components of said panel are secured together over their entire surface area.
 11. An arrangement as defined in claim 1 wherein there is a second main sheet constituting another one of said components, said first sheet is in facing relationship to said second sheet, and said second sheet is arranged to be flexed toward said first sheet, by a flexing force acting against the face of said second sheet which is directed away from said first sheet, to cause said second sheet to impose flexing forces on said first sheet sufficient to break said first sheet before said second sheet breaks.
 12. An arrangement as defined in claim 1 wherein said first sheet is held in said panel in a state of elastic flexure such that its outer face is in a state of tension or reduced compression.
 13. An arrangement as defined in claim 1 wherein there is a second main sHeet constituting another one of said components, said first sheet is in facing relationship to said second main sheet and said second sheet is of plastic material.
 14. An arrangement as defined in claim 1 wherein said components include in addition to said first sheet, only one second main sheet of an at least substantially vitreous material, and said first sheet is in facing relationship to said second main sheet.
 15. An arrangement as defined in claim 14 wherein the relative inherent strengths of said first and second sheets are such that, independently of each other, the resistance of said first sheet to breakage by flexure subjecting its said one face to tensioning forces is higher than the resistance of said second sheet to breakage by flexure which subjects to tensioning forces the face of said second sheet which faces said first sheet in the assembled panel.
 16. An arrangement as defined in claim 14 wherein said second sheet is of glass and over at least one region of said second sheet its face which faces inwardly in said panel is substantially free from flaws liable to cause stress concentrations when tensioning forces are applied to said inner face of said second sheet.
 17. An arrangement as defined in claim 14 wherein external layers of said second sheet at said inner face are compressively stressed.
 18. An arrangement as defined in claim 17 wherein said compressive stressing of said external layers at said inner face of said second sheet is due at least in part to said second sheet being chemically tempered.
 19. An arrangement as defined in claim 17 wherein said second sheet is held in said panel in a state of elastic flexure in a direction such that compressive stresses are present in said external layers of said second sheet due to such flexure.
 20. An arrangement as defined in claim 19 further comprising a frame in which said sheet components are held in such a manner that said second sheet is held in said state of elastic flexure.
 21. An arrangement as defined in claim 19 wherein said second sheet is held in said state of elastic flexure at least in part by said first sheet.
 22. An arrangement as defined in claim 14 wherein said second sheet is chemically tempered, the tensile strength of at least one region of said second sheet, expressed in terms of the tensile force, imposable by flexing said second sheet, which can be sustained by the compressively stressed external layers in the face of said second sheet facing away from said first sheet in said panel, being less than the tensile force necessary for reducing such compressive stresses to zero.
 23. An arrangement as defined in claim 22 wherein said second sheet bears surface abrasions over substantially the whole of the face thereof facing away from said first sheet in said panel.
 24. An arrangement as defined in claim 22 wherein second sheet bears surface scratches over the face thereof facing away from said first sheet in said panel.
 25. An arrangement as defined in claim 22 wherein said first and second sheets have said lower tensile strengths at regions which are in registry with each other in said panel.
 26. An arrangement as defined in claim 14 further comprising at least one intervening sheet of organic material securing said first and second sheets together.
 27. An arrangement as defined in claim 26 wherein said intervening sheet is of a thermoplastic selected from the group consisting of polyvinyl butyral, polyvinyl chloride and a polycarbonate of a bis-phenol and having each of said first and second sheets attached to a respective surface thereof.
 28. An arrangement as defined in claim 26 wherein said intervening organic sheet is arranged to resist penetration or tearing when at least one of said first and second sheets breaks under the impact of a rounded object weighing 10 kg dropped from a height of about 620 cm.
 29. An arrangement as defined in claim 14 wherein said second sheet is thinner than said first sheet.
 30. An arrAngement as defined in claim 29 wherein said first sheet has a thickness in the range of 1.5 to 4.0 mm and said second sheet has a thickness in the range of 1.0 to 2.5 mm.
 31. An arrangement as defined in claim 1 further comprising a covering sheet covering said one face of said first sheet, said covering sheet not being a strength member. 